1. If input conditions specify a range of values, create one valid and one or two
invalid equivalence classes. In the above example, this is (1) less than 50/invalid;
(2) 50 or more/valid.
2. If input conditions require a certain value (for example R and L for the side in our
train example), create an equivalence class of the valid values (R and L) and one
of invalid values (all other letters other than R and L). In this case, you need to
test all valid values individually and several invalid values.
3. If input conditions specify a member of a set, create one valid and one invalid
equivalence class.